In-the-Spectacle-Lens Telescopic Device for Low Vision

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polarizing beam splitter with minimal loss. Because of two passes through the quarter waveplate associated with the second mirror, the light will then be reflected at the second beamsplitter. Thus the total light loss can be limited to 50% in an ideal situation. In practice, bothreflection and transmission factors can be only about 80% efficient for the selected polarizationstate. After two reflections and two transmissions through the beam splitters, a total loss of lightwould be about 80% from natural light (0.5 × 0.8 4 ). Because the eye’s sensitivity is logarithmic,80% loss of brightness should not significantly impact the functionality of the telescope. Thelight loss can be compensated for, in part, by increasing the width of the objective lens providingmore light collection, as discussed below.An additional advantage of the beam splitter design is that the semitransparent beamsplitter is less visible than a regular mirror, which improves the cosmetic appearance of thedevice. However, this advantage holds only for the objective; the ocular beam splitter requiresan opaque backing to improve the contrast of the image for the wearer. The reduced visibility ofthe objective means that it can be made larger and brought closer to the ocular, as it will notblock the view through the carrier lens. Using a semi-transparent mirror also removes theconstraint that the objective mirror be placed farther from the line of sight of the wearer, thusallowing the carrier lens to be made smaller. A smaller carrier lens reduces system weight, and isalso currently more fashionable. The ability to see through the objective area also allows for awider objective, which can in part compensate for the light loss at the first beam splitter.The opaque occluder for the ocular beam splitter can be achieved by providing a polarizeracross the whole front of the lens. The polarizer will turn opaque for the ocular lens but willremain transparent for the objective lens. The light attenuation through the polarizer across thewhole carrier lens may provide a glare control which will increase both the visual comfort of thewearer and the relative brightness of the view through the telescope (as compared to the viewthrough the carrier lens).18
A challenge to the beam-splitter based approach, however, is that any birefringence in thecarrier lens material can rotate the polarization of the light. The rotated light will then bepartially reflected, rather than transmitted, in the first pass through the second beam splitter andwill reduce the light efficiency of the system. Thus the carrier lens has to be as free as possiblefrom birefringence effects.To convert the astronomical Keplerian telescope design using beam splitters (Fig. 10) to aterrestrial setup requires an erecting system. Fig. 11 illustrates one such terrestrial designconstructed by rotating the two assemblies around axes perpendicular to the carrier lensultimately placing the spherical mirrors above the beam splitters. This design, similar to thebasic Keplerian design (Fig. 4), uses the beam splitters as part of the image erecting system.Importantly, this design preserves the property that the width of the fields and the size of theobjective are not limited by the thickness of the carrier lens.19
Page 6: of an object together with the unma
Page 9: The afocal condition is achieved wh
Page 12: wearer an unobstructed, non-magnifi
Page 17: implements a magnifying element sim
Page 21 and 22: ocular elements we used off-the-she
Page 23 and 24: a 5 mm thick carrier lens). Mirrors
Page 25 and 26: 4. Additional considerations4.1. Fi
Page 27 and 28: horizontal extent of the exit pupil
Page 29 and 30: A side effect of using a curved car
Page 31 and 32: 3. R. Lund and G.R. Watson, The CCT

In-the-Spectacle-Lens Telescopic Device for Low Vision

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